A plasma display panel (hereinafter also referred to as “PDP”) has the capabilities of high definition reproduction of pictures and a larger screen, and thus has been commercialized as 100-inch class television sets. In recent years, it has been attempted to apply the PDP to high definition television sets with twice or more scan lines as those of the conventional NTSC TV format. Furthermore, there have been increasing requirements for the PDP to have decreased power consumption for addressing the energy issue and to have lead-free material for meeting environmental requirements.
The PDP is basically composed of a front panel and a rear panel. The front panel is disposed at the front such that it faces the viewer. Such front panel is generally provided with a glass substrate, display electrodes (each of which comprises a transparent electrode and a bus electrode), a dielectric layer and a protective layer. Specifically, (i) on one of the principal surfaces of the glass substrate (e.g. sodium borosilicate glass substrate), the display electrodes are formed in the form of stripes; (ii) the dielectric layer is formed on the principal surface of the glass substrate so as to cover the display electrodes and to serve as a capacitor; and (iii) the protective layer (e.g. MgO layer) is formed on the dielectric layer so as to protect the dielectric layer.
The rear panel is generally provided with a glass substrate, address electrodes, a dielectric layer, partition walls and phosphor layers (i.e. red(R), green(G) and blue(B) fluorescent layers). Specifically, (i) on one of principal surfaces of the glass substrate, the address electrodes are formed in the form of stripes; (ii) the dielectric layer is formed as a base dielectric layer on the principal surface of the glass substrate so as to cover the address electrodes; (iii) a plurality of partition walls (i.e. barrier ribs) are formed on the dielectric layer at equal intervals; and (iv) the phosphor layers are formed on the dielectric layer such that each phosphor layer is located between the adjacent partition walls.
The front panel and the rear panel are opposed to each other so that their electrodes face each other. The opposed front and rear panels are sealed together to form an airtight discharge space that is divided by the partition walls. The discharge space is filled with a discharge gas such as neon (Ne)-xenon (Xe) gas. In operation of the PDP, ultraviolet rays are generated in the discharge cell upon selectively applying a voltage (i.e. voltage of picture signal), and thereby the phosphor layers capable of emitting different visible lights are excited. As a result, the excited phosphor layers respectively emit lights in red, green and blue colors, which will lead to an achievement of a full-color display.
The PDP is ordinarily operated by such a method that sets an initialization period during which charges on the wall are adjusted into a state that allows easy writing, a writing period during which writing electric discharge is carried out in accordance to the input picture signal, and a sustain period during which the pictures are displayed by causing sustain electric discharge in the discharge space wherein the writing operation has been done. Thus, the PDP displays gradation pictures by repeating a period (sub-field) that consists of the periods described above a plurality of times within a period (one field) that corresponds to one frame of picture.
In the PDP, the protective layer of the front panel generally serves to protect the dielectric layer from ion bombardment caused by electric discharge and also serves to release primary electrons for generating an address electric discharge. The protecting of the dielectric layer from ion bombardment is important in terms of preventing the discharge voltage from rising. Whereas, the releasing of the primary electrons for generating the address electric discharge is important in terms of preventing the discharge failure that may cause a blinking of the picture.
There are some attempts to increase the number of primary electrons released from the protective layer, and thereby suppressing the blinking of the picture. For example, some impurity is added to the MgO protective layer, or MgO particles are formed on the MgO protective layer. See Japanese Patent Kokai Publication No. 2002-260535, Japanese Patent Kokai Publication No. 11-339665, Japanese Patent Kokai Publication No. 2006-59779, Japanese Patent Kokai Publication No. 8-236028 and Japanese Patent Kokai Publication No. 10-334809 for example.
The operating voltage of the PDP depends on a secondary electron emission coefficient of the protective layer. Accordingly, it has been proposed to use, as a raw material of the protective layer, an oxide of alkaline earth metal (e.g. calcium oxide, strontium oxide and barium oxide) with lower value of work function than that of magnesium oxide, and thereby decreasing of in an attempt to decrease the operating voltage. However, the oxides of alkaline earth metals have high hygroscopic property, and thus the protective layer made of such oxides tends to absorb the moisture of the atmosphere, and thereby the oxides of the protective layer turn into hydroxide. This results in the instability of the electric discharge. Also the oxides of these alkaline earth metals have tendency to turn into carbonated matter, making it difficult to decrease the operating voltage.
Under the above circumstances, the present invention has been created. Thus, an object of the present invention is to provide a desirable method for producing a PDP in terms of “electric discharge characteristic” and “lower operating voltage”.